The present invention relates to radio frequency circuits, and more particularly to impedance transformation systems for radio frequency circuits.
Inherent problems associated with coaxial cables and twisted wire pairs limit the performance of transformers utilizing these components. More particularly, because of the limited number of coax impedances offered by cable manufacturers, only limited impedance transformations are possible. Also, RF currents flow through both the center conductor and shield of coaxial cable used in transformers. Because of its mechanical structure, the center conductor of the cable has more inductance per unit length than the shield. The additional inductance of the center conductor produces an undesirable phase lag between currents in the center conductor and the shield conductor.
The center conductor of a coaxial cable also has less surface area than the shield. In high power radio frequency (RF) applications, this difference in surface area leads to increased heating of the center conductor relative to the shield. Cooling of the center conductor is also hampered due to the surrounding dielectric, which acts as thermal insulation. In some applications, small diameter coaxial transmission line transformers are immersed in a low dielectric constant, non-conducting liquid to conduct heat away from the cable. This method works well, but requires a large, fluid-tight mechanical enclosure.
It is possible to reduce or eliminate this phase lag problem by cutting the transmission line cable in half and crisscrossing the center conductor and shield. Such configurations provide equal phase lag through each leg of the transformer, but the thermal problems described above may still arise.
Moreover, coaxial transmission line transformers are difficult to manufacture for use in VHF (30 to 300 MHz) and higher frequency applications. Assembly workers and standard coaxial cable stripping machines are not capable of preparing cable having the minimum lead lengths required at VHF and higher frequencies. Parallel (e.g., twisted pair) transmission line can be used, which also reduces the phase and thermal problems. However, most solid-state applications require a characteristic impedance of less than 50 ohms, and it is physically difficult to twist two wires together in such a way to attain such low impedances. Twisted pair transmission lines also have leakage problems at VHF and higher frequencies.
Microstrip transmission line can also be used to make RF transformers and baluns. A microstrip transmission line is a double-sided circuit board having a fixed ground plane on one side and a conductive trace on the other. The board between the ground plane and the conductive trace is made of a dielectric material. Parallel transmission lines must not be run in close proximity to any ground reference plane, so in amplifier circuits having horizontally-mounted microstrip transmission line transformers, heat sinks must be milled under the back side of the transformer to remove the ground reference. This type of construction increases cost and makes inspection after assembly impossible.
Transformers are also used in splitters and combiners for amplifier modules. In one known apparatus, for example, a plurality of 50-ohm amplifier modules are combined utilizing a tree of 2-way zero degree combiners or zero degree hybrids. The microwave equivalent of such a combiner or hybrid is widely known as the xe2x80x9cMagic T.xe2x80x9d This component has two input ports having impedance Z0, each having exactly the same phase (i.e., zero degree delta). A common or sum port has an impedance of Z0/2 ohms. An isolated resistor of Z0*2 ohms connects the two input ports. It is possible to split and combine 2, 4, 8, 16 . . . 2n fifty-ohm modules utilizing this topology.
A common problem when utilizing high power resistors is the unwanted distributed shunt C effect of the body of the resistor against the ground plane. This parasitic shunt C results in increased through loss and unwanted heating of the combiner. A series L element is required on each resistor terminal to cancel the shunt C. This element can be implemented using either lumped or distributed elements. Matching out the shunt C effect of the isolation resistor can yield very close to ideal through loss ( less than 0.1 dB above theoretical) in a zero degree hybrid. However, the matching element may limit the bandwidth in some applications.
The circuit configuration of the zero degree hybrid is exactly the same as an unbalanced to unbalanced 4:1 autotransformer. This autotransformer is a 2-port device with an associated impedance transformation ratio and phase shift. The zero degree hybrid is a 3 port device with a zero degree difference between the two input ports. The impedance transformation ratio of the zero degree hybrid is 4:1, i.e., (Z01+Z02) /4=impedance of sum port.
Common construction materials used to build a zero degree hybrid are coaxial transmission line and waveguide. In many high frequency applications, coaxial transmission line hybrids are subject to many or all of the problems listed above for coaxial transmission line transformers.
There is therefore provided, in one configuration of the present invention, a radio frequency transformer board that has a planar dielectric substrate having a first surface, an opposite second surface, and a transformer. The transformer includes a first elongate conductor disposed on the first surface and having a first end and a second end, a second elongate conductor disposed on the second surface and having a first end and a second end. The first end of the first conductor and the second end of the second conductor are disposed proximate an edge of the substrate and spaced apart from one another along the edge. The second end of the first conductor and the first end of the second conductor are electrically shorted to one another proximate the edge of the substrate.
In another configuration, a transformer assembly utilizing the above-described radio frequency transformer is utilized in a transformer assembly. The radio frequency transformer is mounted perpendularly to a surface of a planar RF circuit board having a plurality of electrical contacts disposed thereon. The first end of the first elongate conductor of the radio frequency transformer, the second end of the second elongate conductor of the radio frequency transformer, and the electrically shorted second end of the first and the first end of the second conductor are electrically coupled to the electrical contacts.
Yet another configuration provides a radio frequency transformer board that has a planar dielectric substrate having a first surface, an opposite second surface, and a transformer. The transformer includes a first elongate conductor disposed on the first surface and having a first end and a second end, a second elongate conductor disposed on the second surface and having a first end and a second end. The first end of the first conductor and the second end of the first conductor are spaced apart from one another proximate an edge of the substrate. The first end of the second conductor and the second end of the second conductor are also spaced apart from one another at the edge of the substrate.
Configurations of the present invention provide RF transformations that avoid undesirable phase lags between conductors, and do not experience differential heating of conductors. Moreover, such configurations are more easily manufactured than twisted pair or coaxial transformers, and are easily replicated as printed circuits. In addition, compensation of distributed shunt C effects of resistors in combiners utilizing RF transformer configurations of the present invention is easily accomplished.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.